In the complex scenario of telecommunications networks, a trend has been establishing itself over the last few years to consolidate certain technologies already in use over the last decade (IP and Ethernet) and to simplify the protocol stack, enriching the pre-existing technologies with new functionality (Pseudowire on Ethernet, QoS on MPLS-TE) with the aim of achieving simpler and cheaper-to-maintain networks.
However, there is no lack of innovative solutions in a market continually in search of solutions aimed at curbing network operating expenses (OPEX) and the possibility of quickly introducing new services on the market, the proliferation of which is driven by the widespread migration of many services to IP. ASON/GMPLS (Automatically Switched Optical Network/Generalised Multi Protocol Label Switching) technology represents one of the innovative solutions of recent years, its characteristics of flexibility, dynamism and automation of the offered services blending well with the network operators' emerging needs.
The by now consolidated trend of services offered by operators and ASPs (Application Service Providers) converging on IP and the extensive use of the IP protocol for computer applications has caused a growth in packet traffic, the characteristics of which are difficult to normalize, especially regarding the coexistence of applications with profoundly different requirements.
ASON/GMPLS technology appears appropriate for offering a wideband transport network that is flexible and dynamic at the same time, able to support IP traffic variability because it allows a harmonious use of pre-existing network technologies through the adoption of a control architecture and special signalling, routing, and discovery mechanisms.
For example, it is possible to integrate networks based on packets, time division, wavelength switching, etc. Each of these technologies has characteristics that make full integration a challenge. The introduction of ASON/GMPLS technology simplifies this process, for example enhancing the lower network levels (wavelength or fibre switching, or time division based) with new functionality, such as the dynamic and automatic creation, deletion and modification of circuits. This requires the introduction of new routing mechanisms that are unlikely to represent a simple transposition of those already tested in the world of packets to the world of circuits. The adaptation of these mechanisms is even more difficult when they must handle new degrees of freedom such as in the case of transparent/translucent optical networks, where it is necessary to check that the optical signal does not accumulate excessive degradation along the route found and so be correctly decoded at the receiver.
For the purposes of the present invention, with “transparent optical network” it is intended an optical network where the transmission of the optical signal is independent of the specific characteristics (digital or analogue type, modulation scheme, signal format, bit rate) of the actual data to be transported through the optical layer; with “opaque optical network” it is intended an optical network making use of 3R signal regeneration at (every) intermediate node along a connection; and with “translucent optical network” it is intended an optical network where a signal is made to propagate in the optical domain as long as possible with respect to optical transmission impairments and signal adaptation or wavelength conversion requirements and then it is regenerated making use of 3R signal regeneration.
An example of network node architecture is shown in FIG. 1. It comprises a number of line interfaces I1-I4 equipped with multiplexing/demultiplexing systems M/D1-M/D4 for the wavelengths in output from/input to the device, a (transparent) wavelength switching matrix SM, a set of regenerators 3R that can be accessed by a signal in transit through the node to restore the quality level it had at source, a set of wavelength converters C that can be accessed by a signal in transit through the node if the signal's wavelength is already in use on the output interface, and a set of transponders T for “colouring” (i.e. assigning a well defined wavelength) the signals in input to the node and their accurate specification, for example, in terms of power level.
An example of translucent network is shown in FIG. 2. It comprises an outer layer based on border Optical Digital Cross Connect nodes (B-ODXC) equipped with transponders T and optionally with regenerators 3R and wavelength converters C, an inner layer that comprises core Optical Digital Cross Connect nodes (C-ODXC) optionally equipped with Regenerators 3R and wavelength converters C and client equipments CE.
With regards to the presence of regenerators and wavelength converters on the various network nodes, it should be stated that these could be equipped in varying numbers (in principle, a node could be purely transparent) based on the results of network planning. Furthermore, their availability on the network changes dynamically according to the state of the network itself and the routing method that is presented herein takes this variability into account.
Thus, under certain network load conditions, it could happen that the regeneration and/or wavelength conversion function is not available on all network nodes. In this type of scenario, it is essential to assess the feasibility of new network routes because there is a maximum length that an optical signal can travel without being regenerated. This length depends on many factors, such as, for example, the type and length of fibre, specifications of the switching matrices, bit rate and the number of waves and their wavelengths. Optical signals degrade along the route due to a number of physical phenomena such as attenuation, dispersion, non-linear effects, etc. When degradation prevents correct reconstruction of the signal at the receiver then it is necessary to regenerate the signal. This process involves electronic components that, in the current state of the technology, are quite expensive. For this reason, it would be convenient for network operators to limit the use of these resources and, in general, of all those resources that, due to their high cost, tend to be present on the network in limited numbers and which, in the following, shall be called valuable resources.
In traditional opaque networks (i.e. networks with nodes fully equipped with regenerators) each node regenerates the received optical signal. In this way, the characteristics that the signal had at source are restored, thereby allowing an arbitrary route to be feasible. In networks of this type, the routing mechanisms disregard physical degradation as the feasibility of each network path is guaranteed by design.
To reduce costs, it is possible to build a transparent optical network, or rather one based on nodes that switch the optical signal without its conversion into electrical form. A telecommunication network of this type is limited in terms of size and is hard to run in practice. These limitations are overcome with the introduction of hybrid nodes in the network, or rather of nodes able to switch the optical signals in a transparent manner and also to regenerate them or change the wavelength when required. Nodes of this type introduce significant flexibility into the network (translucent network), whilst at the same time contain costs. A network of this type can nevertheless contain also totally transparent or opaque nodes, coherently with the results of the network planning study.
The method used to find a route and a wavelength is generally called the “Routing and Wavelength Assignment (RWA) problem”. There is much research and study aimed at identifying the best way of resolving this problem, both in a concurrent manner and by uncoupling it into two sub-problems: the routing problem and that of wavelength assignment. Nevertheless, typically transmission degradation is not taken into account. A brief summary of the state of the art in this field is provided below.
In publication Li Bo, C. Xiaowen, K. Sohraby, “Routing and wavelength assignment vs. wavelength converter placement in all-optical networks”, IEEE Optical Communications, pp. S22-S28, August 2003 the authors propose a solution to the RWA problem based on searching for a set of paths between each source-destination pair that are generally the k shortest paths of the link-disjoint type. A routing request is satisfied by choosing a path from the set of those pre-computed and then assigning a wavelength from those available (for example, with the first-fit method). The weight associated with the routings not only depends on wavelength availability, but also on the path length. In addition, the authors suggest taking the position of the wavelength converters into consideration in order to minimize the probability of blockage and suggest the Minimum Blocking Probability First (MBPF) algorithm for solving the RWA problem.
In U.S. Pat. No. 6,538,777 the authors propose an assignment method for routes and wavelengths based on the change in the network's state following a certain routing rather than on the state of the network prior to routing. The objective of this approach is that of obtaining a flexible network state, minimizing the risk that the assignment of a given route eliminates the availability of the only link able to provide a connection between a pair of nodes.
In publication K. Taira, Y. Zhang, H. Takagi, S. K. Das, “Efficient Lightpath Routing in Wavelength-Routed Optical Networks”, ICOIN 2002, LNCS 2343, pp. 291-304, 2002 the authors propose an exhaustive algorithm that resolves the RWA problem. It first resolves the routing problem and subsequently the wavelength assignment problem. Both problems are formulated as routing problems and resolved using shortest-path routing techniques on the corresponding graphs obtained from transformation of the graph associated with the network.
US 2003/0016414 proposes a solution that takes into account the degradation of an optical signal during its propagation across the network. The authors point out that the wavelengths have different performances in terms of distance traveled before their quality drops below a certain level. They propose a method that selects the most suitable wavelength based on the path distance identified during the routing phase. A path that is too long is divided in two or more sub-paths and the method is iteratively applied to the individual segments. In this way, an attempt to use the minimum number of regeneration and/or wavelength conversion resources is made. The method applies to an optical network with switching nodes equipped with wavelength switches, and a set of regenerators and wavelength converters for regenerating and/or modifying the wavelengths.
Xi Jang, Byrav Ramamurthy, “Interdomain dynamic wavelength routing in the next-generation translucent optical Internet”, Journal of optical networking, Vol 3, No 3, March 2004 (CPA) proposes another method that takes the physical degradation of the optical signals into account. This method concerns an iterative procedure based on the Dijkstra's algorithm (which is a real-world implementation of Shortest Path First (SPF) routing algorithm used in internet routing), the cost function of which depends on the length of the link, the available regeneration resources and the number of available wavelengths on the link. The cost function is parameterized with a parameter d, which takes the number of failed iterations L into account when searching for a route. The higher the value of L, the more the solutions whose traversed nodes have a greater availability of regenerators/wavelengths are favoured.
In particular, the document describes an interdomain dynamic wavelength-routing scheme, which distributes interdomain routing computation to domain gateway. The routing computation at the domain gateways is further divided into three functions. First, a domain gateway uses an LRS (Local Routing Schemes) to compute local routes between itself and each interior node in the same domain as well as between itself and each neighboring domain gateway. Second, a next-hop computation function is used to join the alternate local routes of this domain to the alternate routes of adjacent domains to form the next-hop interfaces leading to desired destinations. Finally, a hop-by-hop lightpath selection function uses the obtained local and next-hop routing information to establish interdomain end-to-end lightpaths.